The present invention relates to a hydraulic power booster, for actuating a master brake cylinder of a vehicle brake system, having a booster piston which is axially movable in a cylinder bore of a booster housing and which, with its peripheral surface, confines a pressure chamber connectible to a hydraulic pressure source and, with an end surface, confines a booster chamber that is connectible with the pressure chamber via a first closable valve passage and that is connectible with an unpressurized reservoir via a second closable valve passage, the two valve passages forming a brake valve for regulating the pressure in the booster chamber which is actuable by way of a reaction piston projecting into the booster chamber.
In the power booster disclosed in German published patent application P 32 18 194, the brake valve is arranged in the booster piston and its second valve passage is connected via a supply chamber, disposed between the booster piston and the master cylinder piston, to the unpressurized reservoir of the energy supply system. The piston slide of the brake valve is provided with a central bore through which the booster chamber is connectible alternately to the first or second valve passages of the brake valve. For this purpose, the slide piston of the brake valve has a comparatively large diameter. This has adverse effects on the sensitivity of response during valve actuation and requires a high standard to be met with regard to the manufacturing accuracy of the slide piston and valve bore. In particular in large-scale production, such manufacturing accuracy can only be achieved by considerable effort.
Another power booster for the actuation of a master brake cylinder of a vehicle brake system is disclosed in German published patent application P 34 40 991.2. In this power booster, the first valve passage disposed in the booster piston is formed by a valve seat which is closable by an axially slidable sleeve-shaped valve element. On it's end surface close to the booster chamber, this valve element includes a valve seat which forms the second valve passage and which cooperates with a valve ball in the end surface of the reaction piston. The second valve passage in the sleeve-shaped valve element and a connecting bore in the booster piston are connected to the supply chamber between the booster piston and the master cylinder piston which is in communication with the unpressurized reservoir. This power booster has the shortcoming that the diameter of the brake valve is of comparatively large size, so that large seal diameters with correspondingly great friction force result both at the brake valve and at the booster piston. Another disadvantage is that the brake valve does not have any lost travel at the transition from closing the second valve passage to opening the first valve passage and vice-versa, so that fluttering of the valve element may occur. This is disadvantageous both for the useful life of the brake valve and for the pressure fluid requirement during the operation.